The present invention relates to detection of a conducting fluid between probes and shock prevention in devices powered by AC. Herein AC refers to the voltage supplied by the electric company, typically at 60 hz in US, while ac refers to any other alternating current signal.
Prior approaches rely on the isolation of the source of detecting current from AC to prevent the fluid from being a shock hazard. As long as isolation is maintained the fluid is safe. To prevent an isolating component failure or other short causing the detection circuit to be “live” the approaches in U.S. Pat. Nos. 3,498,085, 4,380,091 and 4,612,949 use a grounded supply as a source of detecting current. If the isolation fails a fuse or equivalent opens maintaining isolation. Herein ground unless qualified refers to the ground of the AC source. Further any node not directly connected to the ground of the AC source is ungrounded.
The approaches in U.S. Pat. Nos. 4,903,530 and 5,493,877 use an ungrounded supply as a source of detecting current relying solely on the isolation of a transformer to prevent shock. If a short causes isolation failure the detection circuit becomes “live”. In both patents a floating supply voltage is directly tied to the fluid meaning contact with the fluid could result in shock or worse should the detection circuit become “live”.
In U.S. Pat. Nos. 4,182,363 and 5,841,028 it is unclear whether the inventor intended to ground the source of detecting current. Neither patent mentions separating the source of the detecting current and the fluid with sufficient impedance to prevent shock. U.S. Pat. No. 4,182,363 uses a direct connection between circuit ground and the fluid which is not safe in the event of isolation failure if the supply is not grounded.
Unfortunately a proper connection to ground can not always be assumed, particularly in consumer installed devices. Installation resulting in an improperly grounded device is so widespread that appliance manufacturers conventionally do not ground control circuits, only the chassis of the device is grounded. The control supply voltages are floating to prevent an improperly grounded chassis from becoming “live” in the event of isolation failure. Prior approaches have not addressed a device in which the sensing circuitry is not grounded and the fluid is connected to a probe, typically the chassis, that is grounded if correctly installed and floating if installed without a proper ground.
Further previous approaches such as in U.S. Pat. Nos. 4,380,091 and 5,841,028 that use ac current through the fluid must generate the ac signal. Generating ac signals requires additional components, complicating circuit design and adding expense.
It is therefore desirable to create a means in addition to, or in place of isolating components to ensure the safety of fluid detection when a source other than a grounded supply is used to detect fluid. Preferably the device should not incur the expense of generating an ac signal for the detection circuit. Further in applications where one probe, typically the chassis is nominally grounded it should allow the sensing circuitry to correctly detect the fluid whether or not the reference probe is grounded.